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RTD Temperature Calculator (PT100/PT1000)

Calculate temperature from PT100 or PT1000 resistance — enter measured ohms, get °C instantly. Uses the Callendar-Van Dusen linear approximation. Includes presets for common PT100 values (119.4 Ω = 50 °C, 138.5 Ω = 100 °C).

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Formula

T=R−R0R0⋅αT = \frac{R - R_0}{R_0 \cdot \alpha}
T— Temperature (°C)
R— Measured resistance (Ω)
R₀— Nominal resistance at 0°C (Ω)
α— Temperature coefficient of resistance (°C⁻¹)

How It Works

This calculator converts RTD resistance to temperature using the Callendar-Van Dusen equation, used by process engineers, calibration technicians, and industrial automation designers for precision temperature measurement from -200 to +850 C. RTDs (Resistance Temperature Detectors) exploit the linear resistance-temperature relationship in pure platinum: R(T) = R0 * (1 + A*T + B*T^2 + C*(T-100)*T^3) per IEC 60751:2022. The standard coefficients are A = 3.9083e-3 /C, B = -5.775e-7 /C^2, and C = -4.2735e-12 /C^4 (C applies only below 0 C). PT100 sensors (R0 = 100 Ohm) provide 0.385 Ohm/C sensitivity; PT1000 sensors (R0 = 1000 Ohm) provide 3.85 Ohm/C, enabling direct microcontroller ADC interfacing. IEC 60751 defines accuracy classes: Class AA (+/-0.1 C at 0 C), Class A (+/-0.15 C), Class B (+/-0.3 C), and Class C (+/-0.6 C). The temperature coefficient alpha = 0.00385055 Ohm/Ohm/C is the European standard (DIN/IEC); some US sensors use alpha = 0.003916 (ASTM E1137).

Worked Example

Problem

A 4-wire PT100 sensor reads 138.51 Ohm in a pharmaceutical reactor. Calculate the temperature and verify it meets IEC 60751 Class A tolerance.

Solution
  1. Given: R = 138.51 Ohm, R0 = 100 Ohm, IEC 60751 coefficients
  2. For T > 0 C, use quadratic: R = R0*(1 + A*T + B*T^2)
  3. Rearrange: 138.51 = 100*(1 + 3.9083e-3*T - 5.775e-7*T^2)
  4. 1.3851 = 1 + 3.9083e-3*T - 5.775e-7*T^2
  5. Solve quadratic: T = (A - sqrt(A^2 - 4*B*(1-R/R0))) / (2*B)
  6. T = (3.9083e-3 - sqrt(1.528e-5 + 2.224e-6)) / (-1.155e-6) = 100.0 C
  7. Class A tolerance at 100 C: +/-(0.15 + 0.002*|T|) = +/-0.35 C
Result: Temperature is 100.0 C. The reading is within Class A tolerance if the sensor resistance is within 138.51 +/-0.135 Ohm (0.35 C * 0.385 Ohm/C).

Practical Tips

  • ✓Use 4-wire (Kelvin) connection for +/-0.01 C accuracy; 3-wire connection assumes matched lead resistances and achieves +/-0.1 C; 2-wire is only suitable for short runs (<1 m) or when lead resistance is calibrated out per ASTM E1137
  • ✓Limit excitation current to 1 mA to keep self-heating below 0.05 C in typical industrial installations; at 5 mA, self-heating in still air reaches 0.5-2 C depending on sensor construction
  • ✓For temperatures above 500 C, use platinum RTDs with ceramic insulation rated to 850 C; for cryogenic applications below -200 C, use rhodium-iron RTDs per NIST calibration standards

Common Mistakes

  • ✗Using the two-term equation below 0 C omits the cubic C-coefficient, causing errors exceeding 0.5 C at -100 C and 2.5 C at -200 C per IEC 60751 Annex B calculations
  • ✗Confusing PT100 (100 Ohm) and PT1000 (1000 Ohm) values produces 10x temperature errors; verify R0 matches your sensor type before calculation
  • ✗Ignoring lead resistance in 2-wire connections: 1 m of 24 AWG copper wire adds 0.17 Ohm, causing 0.44 C error in PT100 systems (0.044 C error in PT1000)

Frequently Asked Questions

PT indicates Platinum, the sensing element material. The number (100 or 1000) is the nominal resistance in Ohms at 0 C per IEC 60751. Platinum is chosen for its chemical stability, reproducible resistance-temperature relationship, and wide operating range (-200 to +850 C). The temperature coefficient alpha = 0.00385055 Ohm/Ohm/C is standardized internationally, ensuring interchangeability between manufacturers.
PT100 has R0 = 100 Ohm and 0.385 Ohm/C sensitivity; PT1000 has R0 = 1000 Ohm and 3.85 Ohm/C sensitivity. PT1000 provides 10x better resolution for direct ADC interfacing (a 10-bit ADC achieves 0.08 C resolution vs 0.8 C for PT100 on 3.3 V supply) and reduces lead resistance errors by 10x. PT100 remains the industrial standard due to legacy instrumentation; PT1000 is preferred for new embedded designs per sensor manufacturers Honeywell and TE Connectivity.

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